Summary of chemistry doctoral thesis: Synthesis and characterization of doped Mn, Ce and C to ZnO nanoparticles and evaluation of their photo-oxidation potentiability

The objectives of the thesis: The thesis focused on the synthesis of Mn, Ce, C doped ZnO nanoparticles materials and doped ZnO with multi-layered carbon nanotube nanocomposite materials and assessed their photo-oxidative potentiality through photocatalytic reactions of methylene blue (MB) decomposition in aqueous solution under visible light.

GRADUATE UNIVERSITY SCIENCE AND

TECHNOLOGY -

LUU THI VIET HA

SYNTHESIS AND CHARACTERIZATION OF

NANOMATERIALS Mn, Ce AND C DOPED ZnO AND EVALUATION OF THEIR PHOTO-OXIDATION POTENTIAL

Major: Inorganic chemistry Code: 9.44.01.13

SUMMARY OF CHEMISTRY DOCTORAL THESIS

Hanoi - 2018

Adviser 1: Assoc Prof Dr Luu Minh Đai

Adviser 2: Assoc Prof Dr Dao Ngoc Nhiem

Thesis can be found in

The library of the Graduate University of Science and Technology, Vietnam Academy of Science and Technology

INTRODUCTION The urgency of thesis

Today, the rapid growth of industries is in parallel with the level of serious environmental pollution, especially water pollution The textile industry is one of the industries that causes bad water pollution due to direct discharge of wastewater into rivers The World Bank estimated that from17

% to 20 % of industrial water pollution came from dyeing and fabric processing plants which cause alarming number for textile manufacturers,

as well as environmental managers and scientists So far, the methods of treating textile wastewater are used such as coagulation, sintering, biodegradation, adsorption by activated carbon, oxidation methods Among these methods, the biodegradation is widely applied to treat textile wastewater on a large scale However, under anaerobic condition, azo dye can be reduced to byproducts as the very toxic aromatic amines Recently,

we have found out advanced oxidation method which is a new and promising method for treating textile and dyeing wastewater This method usually uses a catalyst as photocatalysts to generate OH radicals , under illumination which have strong oxidizing ability and can breakdown most organic chemicals

Photocatalysts are oxides such as TiO2, ZnO, SnO2, WO2 and CeO2, which are abundant in nature and are widely used by heterogeneuos processes Among them, ZnO is considered to be a promising catalyst for decomposition of organic pigments as well as for water disinfection The photocatalytic ability of ZnO is higher than that of TiO2 and some other semiconductor oxides on the basis of absorption of solar radiation energy However, ZnO has a relatively large (3,27eV) bandgap energy, which corresponds to the ultraviolet light zone for optimum photocatalytic efficiency

Meanwhile, ultraviolet light accounts about 5 % radiation of solar Therefore, practical application of ZnO has limited In order to improve photocatalytic activity and expand application field, it is necessary to transform the electron properties in ZnO nanostructure and reduce bandgap energy and electron recombination dynamics and optical hole properties DopedMetallic or non-metallic or co-doped metallic and non-metalic to the ZnO is one of the effective methods to increase photocatalytic activity of ZnO Thus, the research topic of the thesis "Synthesis and characterization

of doped Mn, Ce and C to ZnO nanoparticles and evaluation of their

photo-oxidation potentiability" was selected with the following objectives and contents:

1 The objectives of the thesis:

The thesis focused on the synthesis of Mn, Ce, C doped ZnO nanoparticles materials and doped ZnO with multi-layered carbon nanotube nanocomposite materials and assessed their photo-oxidative potentiality through photocatalytic reactions of methylene blue (MB) decomposition in aqueous solution under visible light

2 The content of the thesis:

2.1 The material synthesis:

- Synthesis of Mn doped ZnO and Ce doped ZnO nanoparticle materials

by combustion and hydrothermal method;

- Synthesis of C, Mn and C, Ce co-doped ZnO nanoparticles by hydrothermal method;

- Synthesis of C, Ce co-doped ZnO combined with multi-layer carbon nanotube composite materials

2.2 Physical properties and characteristic studies of the synthesized materials:

Synthesized materials were investigated using thermo-gravimetric and differential thermal analysis (DTA-TG), X-ray diffraction (XRD) analysis, Fourier-transform infrared spectroscopy (FT-IR), UV-Vis diffuse reflectance spectroscopy (UV-VIS), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscope (XPS), Brunauer-Emmett-Teller (BET)analysis and energy –dispersive x-ray spectroscopy (EDX)

2.3 Study of photo-oxidative potentiality of the materials:

Synthesized materials were evaluated for photo-oxidative potentiality through photocatalytic reactions of methylene blue (MB) decomposition in aqueous solution under visible light

1.3 Doped ZnO materials

1.3.1 Doped ZnO materials

1.3.2 Reseaches of ZnO and doped ZnO photocatalytic materials

1.4 Photocatalyst

1.4.1 ZnO photocatalysis

1.4.2 Doped ZnO photocatalysis

CHAPTER 2: EXPERIMENTS AND RESEARCH METHODS 2.1 Synthesis of materials

2.1.1 Synthesis of Mn doped ZnO and Ce doped ZnO

1.1.1.1 Synthesis of Mn doped ZnO and Ce doped ZnO by combustion method (Mn-ZnOĐC)

a> Synthesis of Mn doped ZnO by combustion method

1,32 grams of PVA were dissolved completely in 40 ml of distilled water at 50oC, then cooled to room temperature and added slowly 9,9 ml

of Zn(NO3)2 1M solution and 0,1 ml of Mn(CH3COO)21M solution (molar ratio Mn2+/ (Zn2+) = 1:100) The solution was stirred on the magnetic stirrer Distilled water was added and adjusted pH = 4 by ammonium acetate buffer The solution was stirred continuously on the magnetic stirrer for about 4 hours at 80°C until the water is evaporated completely The gel obtained is clear, transparent and viscous The gel was aged about 12 hours

at room temperature anddred at 100oC and heated at 500oC to obtain gray powder

The surveys on the conditions of Mn-ZnOĐC materials were synthesized

- Effects of temperature: Experimental process was synthesized as above, the samples were calcited at different temperatures, from 300 to 800oC in the air with the heating ramp of 8oC/min

- Effects of pH: The experiments werethe same conditions with different

pH The pHs of the reaction solution were changed 3, 4, 5 and 6 respectively

- Effects of doped manganese concentration: The molar ratios of Mn2+/

Zn2+ were 1%; 3%; 5% and 8% respectively

- Effect of PVA content: The molar ratios of 𝑃𝑉𝐴

𝑍𝑛2++𝑀𝑛2+the synthesized

of samples were 1/1; 2/1; 3/1 and 4/1 respectively

- Effect of temperature of gel formation: The temperatures of gel formation were 50°C, 60°C, 80°C and 95°C, respectively

b> Synthesis of ZnO doped by Ce by combustion method

The synthesis of Ce doped ZnO(Ce-ZnOĐC) was the same as that of the Mn-ZnOĐC synthesis Salt Mn(CH3COO)2was changed by salt Ce(NO3)3

2.1.1.2 Synthesis of Mn doped ZnO and Ce doped ZnO by hydrothermal method

a>Synthesis of Mn doped ZnO doped (Mn-ZnO)

0,664 g of Zn(CH3COO)2.2H2O and an amount of Mn(CH3COO)2.4H2O salt (the molar ratio of Mn2+/Zn2+ 2%) were dissolved in 75 ml of C2H5OH to obtain the solution A 0,4g of NaOH were dissolved in 75ml of distilled water to obtain solution B Solution A was added gradually to solution B and stirred for 1,5 hours The whole mixture was transferred to the autoclave and heat in the oven at 150°Cfor

24 hours cooled to room temperature, filtered and rinsed twice with distilled water and ethanol and dried the final product at 80°C for 10 hours

to obtain a solid powder

The survey on the conditions of material was synthesized

- Effects of the temperature: The temperatures of hydrothermal were changed from 110°C to 170°C

- Effects of NaOH concentration: The molar ratios of NaOH/Zn2+were changed 1.5; 3.0 and 6.0

- Effect of solvent: The volume ratio (ml) of water/ethanol (H/R) solution were 150/0, 70/80, 110/40 and 40/110

- Effect of doped manganese content: The molar ratios were Mn2+/Zn2+were 1%, 2% and 6%, respectively

- Influence of PVA content: Experiments carried out were 7%, 10% and 15%, respectively

b> Synthesis of Ce doped ZnO by hydrothermal method

The synthesis of Ce-ZnO was prepared similarly to the Mn-ZnO synthesis Salt Mn(CH3COO)2 was replaced by salt Ce(NO3)3

2.1.1.3 Synthesis of C, Mnco- doped ZnO and C,Ce co-doped ZnO by hydrothermal method

a> Synthesis of C and Mn co-doped ZnO by hydrothermal method (C, ZnO)

Mn-0,664 g of Zn(CH3COO)2.2H2O and 0,015 g of Mn(CH3COO)2.4H2O salt (the molar ratio of Mn2+/Zn2+ 2%) were dissolved in 60 ml of C2H5OH to obtain the solution A 0,0132 g of PVA (the molar ratio of PVA/Zn2+ =10%) were dissolved in 30 ml of distilled water to obtain solution B 0,4 g of NaOH (the molar ratio of NaOH/Zn2+

= 3) were dissolved in 60 ml of C2H5OH to obtain solution C Solution A was added gradually to solution B and stirred for 0,5 hours Then, solution

C was added gradually and stirred for 1 hours This mixture was transferred

to the autoclave and heat in the oven at 150°C for 24 hours After, the autoclave was cooled to room temperature The thermal products were

filtered and rinsed twice with distilled water and ethanol and dried the final product at 80°C for 10 hours to obtain a solid powder

The survey on the conditions for material was synthesized

- Effects of the temperature: The temperatures of hydrothermal were changed from 110°C to 170°C

- Effects of NaOH concentration: The molar ratios of NaOH/Zn2+were changed 1.5; 3.0 and 6.0

- Effect of solvent: The volume ratio (ml) of water/ethanol (H/R) solution were 150/0, 70/80, 110/40 and 40/110

- Effect of doped Manganese content: The molar ratios were Mn2+/Zn2+were 1%, 2% and 6%, respectively

- Influence of PVA content: Experiments carried out were 7%, 10% and 15%, respectively

b> Synthesis of C,Ce co-doped ZnO (C, Ce-ZnO)

The synthesis of C,Ce-ZnO was prepared similarly to the C,Mn-ZnO synthesis Salt Mn(CH3COO) 2 was replaced by salt Ce(NO3)3

2.1.3 Synthesis of C, Ce co-doped ZnO combined with multi-layer carbon nanotube composite materials (C,Ce-ZnO/MWCNTs)

a>Preperation and treatment of multi-layer carbon nanotubes (MWCNTs)

The mixture of 5 grams of carbon (MWCNTs) was boiled with 500ml by concentrated HNO3 solution in the circulation system about 3 hours MWCNTs were rinsed by deionized water to pH = 7 and dried at

90oC for 12 hours and continued to boil it by 500ml NaOH 0.5M for 3 hours in the circulation system and cooled and rinsed several times with deionized water to pH approximately 7 and dried at 90oC for 12h

b> Synthesis of C,Ce-ZnO/MWCNTs nanocomposite materials

The condition to prepare C,Ce-ZnO/MWCNTs materials was the

same as that to prepare C, Ce-ZnO, The difference in reaction condition is the of weight percentages of MWCNTs/materials of approximately 16.94%; 22.22%; 37.97%; 50.5% and 62.01%, respectively denoted CZCT1, CZCT2, CZCT3, CZCT4 and CZCT5 were added after solution was stirred 60 minutes

2.2 The methods to study materials

Characteristic properties of materials were investigated using thermogravimetric and diferential themal analysis (DTA-TG), X-ray diffraction (XRD) analysis, Fourier-transform infrared spectroscopy (FT-IR), UV-Vis diffuse reflectance spectroscopy (UV-VIS), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray

photoelectron spectroscope (XPS), Brunauer-Emmett-Teller (BET)analysis and energy –dispersive x-ray spectroscopy (EDX)

2.3.Evaluation of photocatalytic activities of materials:

2.3.1.Photocatalytic reactions of methylene blue (MB) decomposition

of materials under visible light

a >Caribration curve results of MB solution

The caribration curve was created and identified by linear interval of

MB solution

b> Perform photocatalytic reactions

0.1 g of material were reacted in 100 ml of MB solution under visible light (from Osram and Ace glassphotochemical equipment- sunlight simulated photocatalytic device) First, the mixture was stirred in the dark until the solution was in absorptive balance 3 or 4 ml of MB solution was centrifuged to extract the solids and measure the optical density (time t =

0, optical density Ao) The solution was illuminated and stirredcontinuosly For each period of time, 4 ml of sample were centrifuged and measured density (At) until the solution was discolored

Decompositon efficiency MB was calculated with the formula:

2.3.2 Determination of photodynamic kinetics

Langmuir - Hinshelwood model were used to test the photocatalytic kinetic reactions The kinetic equation is ln (Co/Ct) = kapp.t

Where Co and Ct are the reactant concentrations at times t = 0 and t = t respectively, kappis constant of speed of reactions

2.3.3 Measurement method of chemical oxygen demand (COD) TCVN 6491: 1999

COD was determined by accordance with TCVN 6491: 1999 (ISO 6060: 1989) and SMEWW 5220 standardized methods for analysis water and waste water

CHAPTER 3: RESULTS AND DISCUSSIONS

3.1 The synthesis of Mn doped ZnO and Ce doped ZnO materials

In this section, Mn doped ZnO and Ce doped ZnO materials were synthesized by two different methods: combustion and hydrothermal method The effects of factors on the structure, phase composition and average crystal size of the materials were investigated by using X-ray diffraction (XRD) method The characterized properties and photocatalytic activity of representative materials (materials that had been synthesized at

the best conditions within the scope of the thesis review) were studied by using modern physic-chemical methods

3.1.1 X-ray diffraction (XRD) results

Fig 3.1 The XRD paterns of (a) Mn-ZnOĐC, (b) Ce-ZnO ĐC, (c)

Mn-ZnOTN and (d) Ce-ZnOTN with the different doping content

of Mn and Ce

(b) (a)

(d)

(c)

The effects of doped Mn and Ce on the structure, crystal phase composition of the synthesized materials were focused The Mn doped ZnO and Ce doped ZnO materials synthesized by the combustion and hydrothermal method were signed by Mn-ZnOĐC,Ce-ZnOĐC and Mn-ZnOTN, Ce-ZnOTN

The results of XRD (fig 3.1a, 3.1c, 3.1d) were shown that all of the Mn-ZnOĐC, Mn-ZnO-TN and Ce-ZnOTN materials and the samples with the mole ratio Ce3+/Zn2+ ≤ 2(fig 3.1b) had hexagonal wurtzitesingle phase structure of ZnO However, the XRD patterns of Ce-ZnOĐC with the molar ratio of Ce3+/Zn2+ ≥ 3 (Fig 3.1b) were shown that there were a cubic phase

of Ce or CeO2 It was remarkable that the XRD diffraction peaks of these materials were shift slightly towards smaller 2 theta diffraction angle as compared to XRD patern of ZnO synthesized at the same condition It was suggested that Mn and Ce were doped into ZnO due to interstitial incorpration of Ce ions in ZnO matrix or replaced by Zn ions with Mn ions

in ZnO crystal lattice Ion Zn2+ and ion Mn2+ had the same charge and their radius were not significantly different (Zn2+: 0.74Ao, Mn2+: 0.8Ao) Therefore, Mn2+ions can be subtituted Zn2+ ions or occupied defects during the synthesis process Radius of Ce ions (Ce4+: 0.92Ao, Ce3+: 1.03Ao) were bigger and more different that of Zn ions so Ce ions can be successfully intergrated into the Zn ions sites.Cerium ions were easily reduced or oxidized to form Ce and CeO2 because of the short reaction time (2 hours) and high heating temperature (550oC)(XRD patern 3.1b)

3.1.2 FT-IR spectra results

Table 3.1 Wavenumber and vibration of the bonds

Ocillated

bonding

Wavenumber (cm -1 )

ZnOĐC ZnOĐC Ce- ZnOTN Mn-

Mn-ZnOTN

Hình 3.2 FT-IR spectra of Mn-ZnOĐC, Ce-ZnOĐC, Mn-ZnOTN

and Ce-ZnOTN

The results of the FT-IR spectra (Figure 3.2 and Table 3.1) of the synthesized materials were confirmed the presence of the bonds in the samples Especially, the Zn-O-Mn and Zn-O-Ce bonds and wavenumbers

of Mn-ZnO and Ce-ZnO materials were shifted toward larger wavenumber compared with Zn-O bond in ZnO material that was synthesized at the same condition

3.1.3 The results of SEM, TEM

The TEM images of Mn-ZnOĐC and Ce-ZnOĐC were shown that the Mn-ZnOĐC and Ce-ZnOĐC materials which synthesized by the combustion method were uniform nanosphere particles Mn-ZnOĐC particles were aggregated together The SEM images of Mn-ZnOTN and Ce-ZnOTN were shown that the particles were uniform nanorods The Mn-ZnO material was short nanorods and the Ce-ZnO was long nanorods

Fig 3.3.SEM images of Mn-ZnOĐC, ZnOĐC, Mn-ZnOTN and

Ce-ZnOTN

3.1.4 The UV-VIS results:

Fig 3.4.Band gap of Mn-ZnO Mn-ZnOĐC, Ce-ZnOĐC, Mn-ZnOTN and

Ce-ZnOTN

The UV-VIS results (Fig 3.4) were demonstrated the successful doping

of Mn and Ce into ZnO matrix As a result, ZnO band gap was narowed

after doping with Mn or Ce compared with the undoped ZnO Band gaps

of Mn-ZnOĐC, Ce-ZnOĐC, Mn-ZnOTN and Ce-ZnOTN were 2.97; 3.0; 3,0 and 3,03 eV respectively coresponded with the optical absorption wavelengths ≤ 418; ≤414; ≤414 and ≤410 nm respectively Therefore, the results of the study and characteristics were shown that Mn doped ZnO and Ce doped ZnO were successfully synthesized by combustion and hydrothermal methods Materials were in nanometer size and had the

ability to absorb visible light

3.1.5 Photocatalytic activity of Mn-ZnO and Ce-ZnO

The MB decomposition abilities of the materials under the visible light were increased by following order: Mn-ZnO-ĐC <Ce-ZnO-ĐC <Mn-ZnO-TN<Ce-ZnO-TN (Fig 3.5) The efficiency of MB decomposition were 58.5%; 73.7%; 88.4% and 98.1% respectively under visible light after

90 minutes (Fig 3.6)

Fig 3.5 The efficiencies of MB under visibe light of (a) ZnOĐC,

Mn-ZnOĐC, Ce-ZnOĐC; (b)ZnOTN, Mn-ZnOTN and Ce-ZnOTN

Fig 3.6.The efficiencies of MB decomposition after 90 minutes under

visibe light of Mn-ZnOĐC, Ce-ZnOĐC, Mn-ZnOTN and Ce-ZnOTN

3.1.6 The kinetics of MB decomposition reactions of Mn-ZnO and ZnO under visible light synthesized by two different methods

Fig 3.7 Relationship between ln (Co / Ct) and time for

photocatalytic degradation of MB under visible light with Mn-ZnOĐC,

Ce-ZnOĐC, Mn-ZnOTN and Ce-ZnOTN

The kinetics of MB decomposition reactions of the materials were complied the Langmuir-Hinshelwood model and a first - order reaction The constant rate k increased in the order: Mn-ZnOĐC<Ce-ZnOĐC<Mn-ZnOTN <Ce-ZnOTN The rate constant of the MB decomposition reaction under the visible light of Ce-ZnOTN was the biggest that of MB decomposition coresponded to the fact The speed of MB decomposition reaction of Ce-ZnOTN is the fastest

Based on the results of the studies about the characteristics and preliminary evaluation of photocatalytic activity of the representative materials, the hydrothermal method was chosen for the synthesis of

Ce-ZnOĐC Y = 0,0141x – 0,0987 0,9983 0,012

Mn-ZnOTN Y = 0,0280x - 0,2157 0,9950 0,025

Ce-ZnOTN Y = 0,0315x – 0,1426 0,9995 0,029

3.2 The synthesis of Mn, C codoped ZnO (C, Mn-ZnO) and ZnO Ce,

C codoped ZnO (C, Ce-ZnO)

As presented by the experimental section in 2.1.2, C, Mn-ZnO and C, Ce-ZnO materials were synthesized by hydrothermal method at different conditions with precursors of zinc acetate, manganese acetate, nitrate, PVA

and the solvents such as ethanol and distilled water In which, PVA and ethanol were considered as carbon sources in the synthesis of materials

3.2.1 X-ray diffraction (XRD) resultsof C, Mn-ZnO and C, Ce-ZnO

The effects of Mn and C doping as well as of Ce and C doping to the structure and crystal phase components of ZnO were discussed

The XRD paterns of C,Mn-ZnO and C,Ce-ZnO (Fig 3.8 and 3.9) were shown that all samples had a single hexagonal structure of ZnO No phase

of Mn, Ce, C as well as their compounds were appeared Notably, the diffraction peak positions of these materials were slightly shifted towards the larger 2θ angles compared with the undoped ZnO which was synthesized in the same condition This was different from Mn doped ZnO,

Ce doped ZnO or C doped ZnO which their diffraction peak positions were shifted towards the lower 2θ angles Therefore, it was shown that manganese and carbon as well as Ce and C were successfully co-doped into crystal lattice without altering the structure of ZnO Manganese and cerium were presented by the precursors of manganese acetate and nitrate nitrate, while carbon could be produced by PVA and ethanol in the hydrothermal process It was precise by the hydrothermal process with high spontaneous pressure, manganese, cerium and carbon can be doped into the ZnO matrix

Fig 3.8 XRD paterns of C,Mn-ZnO at (a) different molar ratios of

Mn2+/Zn2+and (b) different molar ratios of PVA/Zn2+

(a)

(b)